The thread title is disingenuous, John, it isn't the neti pots that are dangerous, it is some water. I've always used tap water and a combination of hot water from my tea kettle. Now I'll use water from the tea kettle that has boiled and had time to cool. I use salt and a little baking soda. Amounts to about a 3/4 to 1 tsp per cup of warm water. Too little and it hurts like hell. The water needs to be saline.
If you have a fully-engaged sinus infection then neti pots aren't recommended, they can make things worse, but if you feel one starting to come on, then using it can make it go away before it gets started. I haven't had to take antibiotics for a sinus infection in several years, since I started using one.